The invention relates to a method for reducing pollutant circulations, particularly of circulations of alkali and chlorine compounds in an apparatus for the manufacture of cement clinker from raw meal which is preheated in at least one train of pipes of a heat exchanger (particularly a cyclone suspension type heat exchanger) permeated by the exhaust of a rotary tubular kiln and is burned into cement clinker in the sintering zone of the rotary tubular kiln. The clinker is cooled in a cooler, connected downstream, with a bypass gas vent, or respectively a gas sub-vent for letting out a pollutant containing sub-stream of the exhaust of the rotary kiln from the rotary kiln inlet chamber, with cooling and de-dusting of the vented gas sub-stream.
It is known in cement technology that many of the requisite materials for cement clinker manufacture contain secondary constituents such as alkali compounds, chlorine, sulfur, heavy metals, etc. which evaporate in the region of the sintering zone as alkali chloride and alkali sulfate compounds, then condense again in the preheating region and thus build circulations which not only adversely affect the quality of the cement clinker, but can also considerably damage the combustion process itself. To suppress an alkali circulation in a cement clinker combustion apparatus, as well as to reduce the alkali content in the cement clinker, it is known to bifurcate and lead away a part of the kiln exhaust containing alkali compounds through what is known as a bypass gas vent, or respectively, sub-vent, at the lower region of the exhaust off take main between rotary tubular kiln and suspension type gas pre-heater, or respectively, calcinator, i.e., at a temperature level at which the volatile constituents are not yet condensed, in order to then subsequently introduce into the gas sub-stream exhaust conduit, raw meal and processing dust as condensation nucleus, as well as a cooling medium such as water and/or outside air, and to allow the pollutants contained in the gas sub-stream to condense at the solid matters (DE-A-32 15 793 as well as, e.g., essay "Probleme der Elektroentstaubung hochalkalichloridhaltiger Abgase aus Teilgasabzugen bei Warmetauscher-Trockendrehofen" in DE-Z "Zement-Kalk-Gips" Nr. 5/1978 pages 236 to 238). If the vented gas sub-stream and the dust drawn out therewith are high in alkali content, then the bypass filter dust separated out of the gas sub-stream must be rejected, as well as the potentially still insufficiently purified gas sub-stream itself.
On the other hand, the development of rotary tubular kilns for cement clinker combustion is directed to the goal of utilizing the shortest possible rotary tubular kilns which are still mounted on two mounting stations. Short rotary tubular kilns result in relatively high kiln exhaust temperatures of, e.g., 1300.degree. C. which lead to a high thermal loading of the kiln inlet chamber. However, it would not be possible to cool the kiln inlet chamber by means of a cooling jacket, precisely because raw meals, which, due to their chemical constituents, tend toward conglutinations, would bake on to the walls in the kiln inlet chamber.